package szml;

import java.util.Scanner;

public class code02 {

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int num = scanner.nextInt();
        System.out.println(count(num));
    }

    private static int count(int num) {
        if (num == 1)
            return 2;
        else if (num == 2) {
            return 3;
        }
        return count(num - 1) + count(num - 2);
    }
}
